In the Koikatsu Studio mode, select your character, navigate to the material editor, and change the shader type from the default option to a xukmi Vanilla Plus shader. Tips for Better Lighting with xukmi.shaders
The xukmi.shaders.fx file is essentially a container or a specific preset that tells the emulator how to process the colors, pixels, and scanlines of the game you are playing. xukmi.shaders.fx